1. Field of the Invention
The present invention relates to a data storage device such as a hard disk drive, and in particular to an apparatus and a method for positioning a head at a target position.
2. Background Art
A hard disk drive has magnetic heads for reading data from a magnetic disk, or for writing data thereto. These magnetic heads are mounted on an actuator mechanism that is driven by a VCM (Voice Coil Motor). When a magnetic head reads or writes data, the actuator mechanism is driven and moves and positions the magnetic head above a predetermined track. For the moving and the positioning of the magnetic head at a predetermined location, servo data recorded on the magnetic disk is employed.
On a magnetic disk, such as one in a hard disk drive, multiple data tracks are concentrically formed, and identification data and a burst pattern are stored in advance in the direction of the diameter of the disk. The identification data represents the track addresses of the data tracks, and based on the identification data read by a magnetic head, the rough position of the magnetic head, i.e., the position of a magnetic head relative to a data track, can be determined. A burst pattern includes multiple burst pattern arrays wherein signal storage areas, the phases of which differ, are arranged at predetermined intervals in the direction of the diameter of the disk. A signal output by a magnetic head in accordance with a burst pattern can be employed to detect even a slight position shift of the magnetic head, i.e., a deviation wherein the position of the magnetic head is shifted away from a data track relative to the magnetic head.
To read data from or write data to a magnetic disk, first, while the magnetic disk is rotating, identification data read from the magnetic head is employed to determine the rough position of the magnetic head, and the magnetic head is driven to move it to a specific data track. Then, a signal output by the magnetic head in accordance with the burst pattern is employed to precisely position the magnetic head relative to the specific data track. Finally, data can be read from or written to the magnetic disk. This process sequence is called a seek mode. Further, the feedback control is performed so that even during the reading or writing of data, based on a signal that is output by the magnetic head in accordance with the burst pattern, the magnetic head can be positioned at a constant location relative to a specific data track. This operation is called a following mode.
In a hard disk drive, a magnetic disk is so provided that it encircles the outer circumference of a spindle that is rotated by a motor. However, due to manufacturing errors, the center of such a magnetic disk tends to be slightly eccentric relative to the rotational center of the spindle. This eccentricity can serve as a feedback control system disturbance that governs the positioning of the magnetic head at a constant location relative to a specific data track.
Specifically, the feedback control system defines, as a feedback element, a signal that is output by the magnetic head in accordance with the burst pattern, and employs the position of the magnetic head relative to a data track, which is obtained from this signal, and a deviation signal, which represents the deviation from the target position of the magnetic head, to generate and output a control signal that matches the positioning of the magnetic head with the target. In this manner, the feedback control system determines the positioning of the magnetic head. However, even when the positioning of the magnetic head can match the target position, because of the above described eccentricity, a disturbance (a repeatable run out, hereinafter referred to as an RRO) that increases the deviation is added at every predetermined cycle. Therefore, the control by the feedback control system can not follow the RRO, so that conventionally the shifting from the target position of the position of the magnetic head exceeds a permissible value. Accordingly, errors occur repetitively; for example, part of the information read from the data track is lost, or part of the information to be written to the data track can not be written normally.
To resolve these problems, one hard disk drive has been proposed that employs a digital peak filter for obtaining a high gain, i.e., an effective gain, only for an element having a specific, comparatively low frequency (e.g., 60 Hz) that is included in a deviation signal. An input deviation signal is input to the digital peak filter, whereat a control signal is generated, by adding a signal produced by the digital peak filter, and is output. For the hard disk drive, since the specific frequency matches the frequency corresponding to an RRO occurrence cycle, the head can follow the data track without changing the configuration of other sections of the control system.
However, an RRO having a high frequency occurs in a hard disk drive, due not only to the eccentricity of the center of a magnetic disk relative to the rotational center of a spindle, but also because the bearing of the motor that rotates the spindle is not circular. An RRO having a high frequency becomes a problem, especially when the pitches at which data tracks are formed on the magnetic disk are reduced (the distances are smaller) in order to increase the data recording density of the magnetic disk. On the other hand, when for the hard disk drive the frequency for increasing the gain of the digital peak filter is simply increased, the stability of the control system is deteriorated.
Further, for a notebook PC, for example, vibration sources, such as a floppy disk drive, a CD-ROM drive, a cooling fan and a loudspeaker, are internally provided. And in addition, depending on the environment in which the notebook PC is used, external vibrations may be transmitted to the hard disk drive. Thus, it is difficult to designate a frequency for all the vibrations that may be transferred to the hard disk drive. And therefore, while a digital peak filter for a hard disk drive may effectively handle a specific, fixed frequency, it is impossible to suppress the whole range of frequencies which produce vibrations that can be conveyed to the hard disk drive.